This project will attempt to define the mechanisms by which the autonomic nervous system alters the electrophysiological properties of the heart in the presence of myocardial ischemia. Electrophysiological studies will be done in intact dogs, with and without myocardial ischemia, and in isolated Purkinje fibers. Cyclic nucleotides levels (cyclic AMP and cyclic GMP) will be measured in the tissues studies and the levels correlated with electrophysiological findings.